What is a potential outcome of cross-immunity?

Cross-immunity refers to the phenomenon where exposure to one pathogen provides some degree of immunity against a different but related pathogen. This occurs due to similarities in the antigens presented by the pathogens, which can trigger the immune system to recognize and respond more effectively to the related pathogen upon subsequent exposure.

When the immune system is activated by the first pathogen, it generates specific antibodies and memory cells that can cross-react with the antigens of the related pathogen. As a result, the individual may experience either a milder form of the illness or may not become sick at all when exposed to the related pathogen due to this pre-existing immunity. This protective effect is often observed in cases where vaccines for one pathogen can also offer some degree of protection against similar pathogens.

In contrast, the other choices do not accurately capture the implications of cross-immunity. Increased susceptibility to infections would suggest a weakened immune response, which is not consistent with the concept of cross-immunity. Infection by multiple pathogens at once could occur independently of cross-immunity and does not reflect the protective nature typically associated with it. Complete immunity to all pathogens is not realistic, as the immune system's response is usually specific and only applicable to narrowly defined antigens rather than a blanket immunity across all pathogens.